Respiratory disease affecting feedlot cattle causes tremendous losses yearly to the cattle industry. Calves are the most severely affected, and a large number of these calves die. This disease is associated with pathogenic microorganisms, particularly Pasteurallae species, and various stresses, such as transportation and overcrowding.
Shipping fever is the most economically important respiratory disease associated with Pasteurella species. The disease is characterized by sudden onset, usually within two weeks of stress. The symptoms include dyspnea, cough, ocular and nasal discharge, inappetence and rapid weight loss, fever, increased lung sounds, immunosuppression, general depression, viral and/or bacterial infection of the lungs. Various bacteria and viruses have been isolated from affected animals including Pasteurella spp., bovine herpes virus 1, parainfluenza-3 virus, bovine-respiratory syncytial virus and Mycoplasma species. The disease typically affects 15-30% of exposed animals and the resulting deaths are typically 2-5% of the exposed population.
Exposure of the animal to stress, plus infection with a variety of viruses, as described above, appears to make the animal susceptible to fibrinous pneumonia caused by P. haemolytica, and to a lesser extent, P. multocida. For a general background on shipping fever see Yates, W. D. G. (1982) Can. J. Comp. Med. 46:225-263.
P. haemolytica also causes enzootic pneumonia and can infect a wide range of animals, in addition to cattle, including economically important species such as sheep, swine, horses and fowl. P. haemolytica is also frequently found in the upper respiratory tract of healthy animals. Pneumonia develops when the bacteria infect the lungs of these animals. Protection against Pasteurella-associated diseases is therefore economically important to the agricultural industry.
There are two known biotypes of P. haemolytica designated A and T. There are also 12 recognized serotypes which have been isolated from ruminants. Biotype A, serotype 1 (referred to hereinafter as "A1") predominates in bovine pneumonia in North America. Shewen, P. E., and Wilkie, B. N. (1983) Am. J. Vet. Res. 44:715-719. However, antigens isolated from different serotypes appear to be somewhat cross-reactive. See, e.g., Donanchie et al. (1984) J. Gen. Micro. 130:1209-1216.
Previous Pasteurellosis vaccines have utilized whole cell preparations of either live or heat killed bacteria of various serotypes as described in U.S. Pat. Nos. 4,328,210, 4,171,354, 3,328,252, 4,167,560 and 4,346,074. Traditional vaccine preparations, however, have not been effective in protecting against Pasteurella infections. Indeed, vaccinated animals are frequently more susceptible to the disease than their non-vaccinated counterparts. Martin et al. (1980) Can. J. Comp. Med. 44:1-10. The lack of protection offered by traditional vaccines is probably due to the absence of important antigens, virulence determinants, or the presence of immunosuppressive components in the preparations.
Other vaccine preparations have included crude supernatant extracts from P. haemolytica. See, e.g., Shewen, P. E., and Wilkie, B. N. (1988) in Can. J. Vet. Res. 52:30-36. These culture supernatants, however, contain various soluble surface antigens of the bacterium and produce variable results when administered to animals. Other preparations include capsular extracts obtained via sodium salicylate extraction (see, e.g., Donanchie et al. (1984) 130:1209-1216; U.S. Pat. No. 4,346,074), saline extracted antigens (see, e.g., Lessley et al. (1985) Veterinary Immunology and Immunopathology 10:279-296; Himmel et al. (1982) Am. J. Vet. Res. 43:764-767), and modified live Pasteurella mutants.
Still other attempts at immunization have included the use of a purified cytotoxin from P. haemolytica. See, e.g. Gentry et al. (1985) Vet. Immunology and Immunopathology 9:239-250. This cytotoxin, which is a leukotoxin, is secreted by actively growing bacteria. Shewen, P. E., and Wilkie, B. N. (1987) Infect. Immun. 55:3233-3236. The gene encoding this leukotoxin has been cloned and expressed in bacterial cells. Lo et al. (1985) Infect. Immun. 50:667-671. Calves which survive P. haemolytica infections possess toxin-neutralizing antibody. Cho, H. J., and Jericho, K. W. F. (1986) Can. J. Vet. Res. 50:27-31; Cho et al. (1984) Can. J. Comp. Med. 48:151-155.
Cytokines are a group of hormone-like mediators produced by leukocytes. Cytokines serve as endogenous signals that act in conjunction with antigens to amplify both localized and systemic host defense mechanisms involving macrophages, lymphocytes, and other cell types. Representative lympokines include interleukin-1 (IL1), interleukin-2 (IL2), interleukin-3 (IL3), interleukin-4 (IL4), and gamma-interferon (.gamma.IFN).
IL1 and IL2 both exhibit thymocyte mitogenic activity and IL2 stimulates T lymphocyte proliferation. IL3 stimulates the growth of hematopoietic progenitor cells and multipotential stem cells, and IL4 acts as an induction factor on resting B cells and as a B cell growth and differentiation factor. IL4 also exhibits T cell stimulatory activity.
.gamma.IFN is predominantly produced by antigen- or mitogen-stimulated T lymphocytes. .gamma.IFN has been shown to be a potent immunomodulator and appears to enhance natural killer cell activity, antibody-dependent cellular cytotoxicity, and cytotoxic T lymphocyte activity (Lawman et al. (1989) "Recombinant Cytokines and their Potential Therapeutic Value in Veterinary Medicine" in Comprehensive Biotech, First Supplement, Animal Biotechnology, Pages 63-106 (Pergamon Press, London).
Gene fusions provide a convenient method for the production of chimeric proteins. The expression of a chimeric protein, such as a cytokine linked to an antigenic polypeptide, allows the simultaneous delivery of both agents to a desired recipient. PCT Publication No. WO 88/00971 (publication date of Feb. 11, 1988) describes the fusion of an IL2 gene with the influenza hemagglutinin coding sequence and the subsequent administration of the fusion protein using a viral vector. The application nowhere contemplates the use of a cytokine fused to leukotoxin for the treatment of pneumonia in animals.